The incorporation of conductive particles in a matrix material to form a conductive composition is in and of itself known. For example, U.S. Pat. No. 4,822,523 discloses an adhesive, electrically conductive, composition consisting of an epoxide resin in which are dispersed particles of silver or other electrically conductive metals such as gold, nickel and copper. Another variation reported in such patent is a conductive composition comprising a para-phenylene polysulfide and silver particles. However, though relatively simple in concept, this technology has apparently been unsatisfactory. The use of solid particles of noble metal, to benefit from their high conductivity, entails a very high cost. Moreover, the matrix compositions can be so rigid that they are incapable of satisfactorily withstanding stresses resulting, for instance, from differential thermal expansion. The aforementioned patent also discloses compositions which are reportedly better able to withstand differential expansion, consisting of a silicone elastomer and silver particles. But, these compositions are also said to be disadvantageous insofar as they reflect a compromise between the competing requirements of maintaining a storage life of satisfactory duration and (on the other hand) optimizing the rate of cross-linking; furthermore, such compositions require the presence of reinforcing fillers (such as fumed silica) which are acknowledged as incompatible with securing satisfactory electrical conductivity.
The art has sought to develop modified conductive filler materials which are, at the same time, acceptably conductive but less expensive than solid noble metal particles. This has led to the popularization of alternatives in which a core of inexpensive metal (such as copper) or inorganic material (such as glass, mineral, etc.) is coated with a conductive metal such as a noble metal, for instance, silver or gold. Also available are composite particles such as aluminum, or noble-metal-coated aluminum, which have embedded therein or otherwise operatively associated therewith a further conductive component such as tungsten carbide or other conductive refractory metal (see, for example, U.S. Pat. Nos. 5,175,056 and 5,399,432).
The patent literature also documents other efforts to address the aforementioned shortcomings. U.S. Pat. No. 4,716,081 contains disclosure of a conductive composition comprising a resin matrix and silver-plated copper particles which have been heat-treated before such incorporation; prior to silver-coating the copper core is pre-treated for several minutes in a bath of silver-complexing, or silver-chelating agent, such as a sodium cyanide or potassium cyanide bath.
U.S. Pat. No. 4,683,082 discloses a polymeric matrix containing conductive particles, and multifunctional agents such as organotin compounds which are utilized on the conductive particles or in the binder system in quantities effective to secure a favorable interface between the conductive particles and polymeric matrix along with satisfactory shelf-life of the organic binder system and electroconductive stability of the particles. And U.S. Pat. No. 5,091,114 discloses an organic resin, such as a thermoset resin, photocurable resin, irradiation-curable resin, thermoplastic resin, or the like, which includes silver-containing particles.
Also, U.S. Pat. No. 5,207,950 is directed to polymer compositions--preferably selected from the group of polyesters, vinyl chloride copolymers, vinylidene chloride copolymers, polyurethanes, ethylene copolymers, acrylate polymers, acrylate copolymers and mixtures thereof--which contain metal-based conductive particles and chlorided metal-based conductive particles. The chlorided particles are subject to a treatment such that the chloride is at least present on the particle surfaces prior to inclusion in the composition; this is typically effected by subjecting a metal-based particle to treatment with a metal-chloride forming solution such as aqueous sodium hypochlorite ferric chloride, sodium chloride or potassium chloride.
Of particular interest are conductive compositions having a silicone-based matrix system. These are valued for their superior elastomeric properties, which enable production of shaped electroconductive bodies suitable for applications which require both high conductivity and high flexibility. Depending on the formulation of the matrix, these can be used to form articles of manufacture such as conductive gaskets, or conductive adhesives.
For example, previously mentioned U.S. Pat. No. 4,822,523 is directed to an electrically conductive composition containing an elastomeric thermoplastic polyblock organopolysiloxane copolymer (formed in a reaction catalyzed by platinum chloride derivatives) and particles with electroconductive properties, such as carbon, titanium carbide, silver, or gold, or a core of any such material or nickel, copper or lead clad with a conductive material.
See also: U.S. Pat. No. 4,777,205 which is directed to an organopolysiloxane composition containing silver-coated mica particles and carbon black, a platinum catalyst, and a platinum-catalyst inhibitor; U.S. Pat. No. 5,227,093 which is directed to an organosiloxane composition (cured with a platinum catalyst) containing silver or silver-coated particles treated with a fatty acid ester prior to their incorporation; and U.S. Pat. No. 4,836,955 which discloses a silicone binder system (having as a first component a vinyl gum type of silicone resin and as a second component a liquid silicone resin) containing silver-plated copper particles for use as a gasket, such formulation exhibiting improved electroconductive stability when the particles are subjected to a long-term heat treatment prior to their incorporation in the matrix; U.S. Pat. No. 5,075,038 which describes a number of different embodiments, such as a siloxane silicone pressure-sensitive adhesive containing silver-plated copper powder, an electrically conductive stock comprising a mixture of siloxane polymer with a curing agent (aminosilane or aminosilazane) and electrically conductive particles having an outer surface of noble metal, vinyl-containing organopolysiloxanes and organopolysiloxanes having Si-bonded hydrogen atoms along with a catalyst and a conductive material, a polymeric composition (e.g., an epoxy resin, silicone, fluorosilicone, polyurethane or long-chain hydrocarbon) loaded with silver flakes, and a silicone composition formed using a platinum catalyst and silicone polymer including amounts of aromatic-containing radicals or ethylenically unsaturated hydrocarbon radicals (e.g., vinyl radicals, in addition to those commonly used as end blockers) as well as silver particles or silver-coated copper, solid glass, hollow glass, mica flakes, nickel granules and spheres, short glass and ceramic fiber.
Further, U.S. Pat. No. 5,229,037 describes multiple systems such as: a highly electroconductive silicone rubber composition containing noble metal powder or an inorganic material (for instance, glass, mica, alumina, carbon and the like) coated or plated with silver, nickel, etc.; a silicone rubber composition obtained by compounding a metallic electroconductivity-imparting agent with an insulating silicone rubber composition in combination with a fine powder of a cured silicone rubber (or alternatively carbon black); and an electroconductive composition comprising a diorganopolysiloxane, spherical particles of a cured silicone elastomer, a metallic electroductivity-imparting agent in finely divided form, and a curing agent, as well as (optionally) a liquid organosilicone compound.
Still other variations are described in: U.S. Pat. No. 5,344,593, which relates to an electroconductive composition comprising a vinyl-group-terminated polydimethylsiloxane, optionally containing other polymer components, a hydrosilyl-group-containing polydimethylsiloxane to act as a curing agent, and electroconductive particles; U.S. Pat. No. 5,384,075, which is directed to a system comprising an organopolysiloxane having at least two alkenyl groups in its molecule, an organohydrogen- polysiloxane having at least two silicone-bonded hydrogen atoms in its molecule, a platinum group metal catalyst, an organosilicone compound having at least one silicone-bonded hydrogen atom and at least one silicone-attached epoxy-group-containing organic group or alkoxy group, and an electrically conductive filler, and (optionally) a flame retardant such as benzotriazole or an addition-reaction-controlling agent such as vinyl-group-containing siloxane; U.S. Pat. No. 5,498,644 directed to an electrically conductive silicone elastomer produced, for instance, by mixing vinylmethylsiloxane having a platinum catalyst with methylhydrogensiloxane and also with silver-coated ceramic microballoons, a Teflon powder and a heat curable silicone rubber system containing an inhibited platinum catalyst; and U.S. Pat. No. 5,091,114 directed to a composition produced by removing water from an emulsion having a dispersed phase of elastomer (e.g., based upon polydiorganosiloxane) and a continuous phase of water, said composition also containing electrically conductive metal coated spheres.
However, despite the proliferation of electroconductive siloxane-based systems as covered above, the technology continues to suffer from a fundamental problem. Though electroconductivity may be acceptable and resistivity low initially, conductive compositions containing conductive particles and a siloxane matrix or the like frequently exhibit substantial increases in resistivity over time, such that within too short a period the compositions are not acceptably electroconductive any longer. This is a very significant drawback, as it means that often within three weeks or less after production, during storage or use, the electroconductive properties of the composition in question are deteriorating. Overcoming the problem by providing a siloxane-based conductive composition which reliably exhibits stable, high electroconductivity for a sustained duration would be a substantial advance of the art.